1. Field of Invention
This invention relates to ink jet printers, and more particularly to method and apparatus for preventing banding defects caused by pre-pulse sequence flip-flop due to temperature fluctuations around critical temperatures when printing in an ink jet printer.
2. Description of Related Art
Generally, a print head temperature fluctuates steadily while printing onto a print medium. As the temperature of the print head varies, the drop mass of an ink droplet fluctuates, which adversely affects the print quality. Conventionally, to reduce the effect of fluctuation in the mass of the ink droplet, a pre-pulse sequence is selectively varied as the print head temperature varies during a printing operation. As the temperature of the print head changes, a particular pre-pulse sequence associated within a particular temperature range is selected. By varying the pre-pulse sequence based on the temperature, fluctuations in the drop mass of the ink droplet can be minimized. In this technique, the boundaries between each temperature range are known as xe2x80x9ccritical temperatures.xe2x80x9d That is, each time the temperature crosses a critical temperature, the pre-pulse sequence is changed. In general, as the temperature rises across a critical temperature, a shorter pre-pulse sequence replaces a longer pre-pulse sequence. This transition causes a sudden drop in the drop mass. The inverse is true when the temperature drops across a critical temperature.
In one example, each temperature range is approximately 5xc2x0 C., and thus the critical temperatures are provided in 5xc2x0 C. increments (e.g., 25xc2x0 C., 30xc2x0 C., 35xc2x0 C, etc.). Thus, a single pre-pulse sequence will be used between the consecutive critical temperatures. While this will result in a slight variation in drop size within each temperature image (e.g., as the temperature increases from 26xc2x0 C. to 29xc2x0 C.), the visual effect is minimal. However, this method does not work well when the temperature of the print head oscillates around a critical temperature.
In particular, small temperature oscillations across a critical temperature (e.g., 35xc2x0 C.) cause one pre-pulse sequence to flip-flop with another pre-pulse sequence associated with a different temperature range, and thus causes frequent large changes in drop mass and objectionable banding. Factors that can cause temporary drops in the detected temperature, which may cause oscillation about a critical temperature include, for example: reducing the pre-pulse train (i.e., due to changing to a new pre-pulse sequence when the temperature rises across a critical temperature), a temporary reduction in print head power due to maintenance operations, and electrical noise associated with the temperature sensor.
The present invention addresses the problems set forth above. Aspects of this invention relate to method and apparatus for preventing banding defects caused by pre-pulse sequence flip-flop due to detected temperature oscillations around critical temperatures in an ink jet printer that uses different pre-pulse sequences depending on the detected print head temperature. Each of the pre-pulse sequences is associated with a corresponding temperature range, the boundaries between adjacent temperature ranges being separated by critical temperatures.
According to one aspect of the invention, rather than merely changing the pre-pulse sequence when the detected temperature crosses a critical temperature, a determination is made as to whether the change in temperature constitutes, or is part of, a temperature change that exceeds a predetermined threshold change amount, prior to determining whether to change the pre-pulse sequence. If the detected change constitutes, or is part of, a temperature change that exceeds the predetermined threshold change amount, then the pre-pulse sequence is permitted to change; otherwise, the pre-pulse sequence is not permitted to change (i.e., it is prohibited from changing). The predetermined threshold change amount functions as a temperature buffer. This temperature buffer can be used for decreases and/or increases in temperature.
In a preferred embodiment, the temperature buffer is used with decreases in temperature because it is temporary temperature decreases that are more prevalent.